Update and improve todo-mode item insertion and editing code

Update and improve todo-mode item insertion and editing code

* lisp/calendar/todo-mode.el (todo-insert-item--param-key-alist)
(todo-insert-item--keyof, todo-insert-item--this-key)
(todo-insert-item--keys-so-far, todo-insert-item--args)
(todo-insert-item--argleft. todo-insert-item--argsleft)
(todo-insert-item--newargsleft, todo-insert-item--apply-args)
(todo-edit-item--param-key-alist, todo-edit-item--prompt)
(todo-edit-item--date-param-key-alist)
(todo-edit-done-item--param-key-alist): Remove.
(todo-insert-item--next-param): Reimplement to take advantage of
lexical binding.
(todo-insert-item): Adjust to new implementation of the above.
(todo-edit-item--next-key): Incorporate now removed global
variables, adjust signature accordingly, update use of pcase.
(todo-edit-item): Adjust to changed signature of the above.